In the past, thermoplastic films and bag structures made therefrom for the storage of food in its refrigerated or frozen condition have employed polyethylene as the material of construction. This type of container or bag is particularly convenient for storage of food and food leftovers which are intended for later reheating or cooking. The storage bags have been formed from comparatively low density polyethylene resin, for example, polyethylene having a density of from about 0.910 to about 0.930 grams per cubic centimeter. However, bags made from such low density polyethylene do not have utility for holding food or food leftovers during heating or cooking because the polyethylene film cannot withstand elevated temperatures of about 215.degree. F. or higher. Even at temperatures below said aforementioned range, polyethylene film has a tendency to at least partially melt and stick to the surface of the heating means or vessel.
In order to overcome such above-indicated drawbacks when using polyethylene films, attempts have been made to employ other resins such as polyester films in elevated temperature environments. However, such films have been found to suffer sealing problems, such as providing narrow heat sealing range, a manufacturing process limitation, and are also expensive for disposable bags. In addition, to compensate for thermal weakness, other thermoplastic resins such as polypropylene which has a melting point of between about 314.degree. F. and 325.degree. F., and wherein the film structure is relatively thick, may be employed at elevated cooking temperatures instead of polyethylene. However, although the melt temperature and microwave ovenable properties of thermoplastic resins such as polypropylene are superior to polyethylene, the major weakness of resins such as polypropylene is their lack of low temperature mechanical strength properties. That is, at or around freezing temperatures, polypropylene films are brittle, the impact strength and puncture resistance of the films is low, and the films are susceptible to development of cracks and pinholes therein. This latter deficiency of polypropylene makes it unsatisfactory for use in the cold storage of foods and the handling of such food containers in the frozen state. That is, when rummaging the containers in a freezer, such handling causes the development of stress cracks in the container walls, and ultimately, leaks develop therefrom when the containers are thawed.
It is also known that food packaging films may be formed from a multilayer film such as a laminate of a polyamide and polyethylene wherein an adhesive binder material or layer is provided between the polyethylene film and the polyamide film layers. However, such multilayer film materials are generally deficient when used as a container which can withstand reheating or cooking temperatures in that the thermal resistance of the polyethylene layer is less than desired.
Thus, it would be desirable to provide thermoplastic films and bag structures that could be used as a food storage bag having improved low temperature impact and puncture strength properties. It would also be desirable to obtain multilayer thermoplastic films and bag structures which will provide improved physical strength properties when the films and containers are employed at elevated temperatures, such as when cooking or heating a variety of foods.